An inverter is used to provide AC current for a motor from a source of DC voltage such as an AC line rectified to energize a pair of DC busses with a potential of several hundred volts. Transistor switches selectively couple each phase of the motor to the positive and negative DC voltage busses for short periods to properly energize each phase of the motor. Pulse width modulation (PWM) is generally used to control each of the switches. When properly controlled this allows the frequency and magnitude of stator voltage applied to an induction motor to be managed so as to maintain constant flux in the motor over a wide speed range and to substantially reduce harmonics in the current supplied to the motor.
It is known to control the inverter switching digitally using a waveform generator in combination with or as part of the code within a microcontroller programmed to control the waveform generator operation on the basis of desired motor operation and feedback information from the motor. The waveform generator then outputs inverter switching signals to energize the motor. The microcontroller affords an operator interface with the inverter. The high potential DC voltage source is usually isolated from the waveform generator by opto-isolators which transmit high frequency switching signals to the transistor switches. Feedback information may be returned to the controller through opto-isolators and linear isolation amplifiers or isolated transducers. Such an arrangement is costly since these components are expensive and many are required. It is proposed then to configure a control having a minimal content of opto-isolators. In particular, it is proposed to provide protection from high voltages by using a serial data link with a pair of opto-isolators to communicate data between the microcontroller and the waveform generator as well as to communicate the feedback signals to the controller. The waveform generator then floats at the high potential of a DC bus and the microcontroller is referenced to ground potential.
Further cost advantages can be had without compromising control efficiency and accuracy by utilizing application specific integrated circuits (ASIC) in conjunction with opto-isolators. The serial communication scheme requires memory capability at the high voltage side of the opto-isolators and this is accommodated in the ASIC along with other serial communication functions, the waveform generator, a utility block including a crystal oscillator and test circuitry, fault logic, a control block to handle other input and output signals, analog/digital conversion, and an output block for generating gating signals to control the transistor switches.